Photodetector and device employing the photodetector for converting an optical signal into an electrical signal

Abstract

An anti-reflective coating having a composite layer of silicon nitride and silicon dioxide may be formed over the entire photosensitive region of the photodetector to minimize the amount of reflection. The composite layer comprises a silicon nitride layer and a dielectric layer contiguous to the silicon nitride layer. The anti-reflective coating may be formed in a CMOS process for fabricating the PN junction in the photodiode and CMOS devices for amplifying the photodetector signal, where the polysilicon gate layer is used as a etch stop. The P+ or N+ material in the PN junction of the photodiode has a distributed design where two portions of the region are separated by a distance in the range of Xd to 2Xd, where Xd is the one-sided junction depletion width, to enhance the electric field and to reduce the distance traveled by the carriers for enhancing bandwidth. A heavily doped region of the opposite type may be added between the two portions to further enhance the electric field. A mask is used to shield a portion of the substrate in which the photodetector region has been or is to be formed when other portions of the substrate region are implanted with a dopant to adjust at least one of the threshold voltages of the other portions. The mask prevents the photodetector region from being affected by such implant.

What is claimed is:  
     
       1. A device for reading recorded information from a surface of a disk, comprising: 
       means for supplying radiation to the surface; and  
       a photodiode for sensing radiation supplied by the supplying means and modified by the disk;  
       said photodiode including:  
       first region of semiconductor material of a first conductivity type;  
       a second region of semiconductor material of a second conductivity type forming a PN junction with the first region;  
       means for applying a reverse bias voltage across the junction;  
       wherein the first region has at least two portions that are spaced apart by a spacing in the range of about 5 to 15 microns, said at least two portions electrically connected to form one side of the PN junction with the second region.  
     
     
       2. The photodiode of  claim 1 , further comprising an anti-reflective filter over the entire first and second regions, said filter comprising a first layer of silicon nitride and a second dielectric layer contiguous with the first layer. 
     
     
       3. The photodiode of  claim 1 , said photodiode further comprising a third region between the two portions of the first region, said third region comprising a heavily doped semiconductor material of a second conductivity type. 
     
     
       4. The photodiode of  claim 3 , said two portions including N+ material and said third region comprising P+ material. 
     
     
       5. The photodiode of  claim 3 , said two portions including P+ material and said third region comprising N+ material. 
     
     
       6. A device for converting an optical signal into an electrical signal, comprising: 
       a semiconductor substrate comprising a surface on one side of the substrate, a photodetector region that provides an electrical signal in response to a light signal, said photodetector region located at or near the surface of the substrate, and said surface comprising a circuit region for processing the electrical signal, said circuit region comprising only CMOS devices; and  
       a device supplying light in a direction towards said surface.  
     
     
       7. The device of  claim 6 , further comprising a capacitor in the circuit region, said circuit region having a polysilicon gate layer, wherein said capacitor comprises a first and a second polysilicon layer, said first polysilicon layer formed together with the polysilicon gate layer, said filter having been formed using as a mask a third polysilicon layer over the photodetector region, wherein said second and third polysilicon layers are formed in the same processing step. 
     
     
       8. The device of  claim 6 , further comprising an anti-reflective filter over the photodetector region of said surface so that light passes through said filter before reaching the photodetector region, said filter located on the same side of the substrate as the circuit region. 
     
     
       9. The device of  claim 6 , further comprising an anti-reflective filter over the entire photodetector region, said filter comprising a first layer of silicon nitride and a second dielectric layer contiguous with the first layer. 
     
     
       10. The photodiode of  claim 9 , said circuit region having a polysilicon gate layer, said filter having been formed using as a mask a polysilicon layer over the photodetector region, said polysilicon layer formed together with the polysilicon gate layer.